I. Field of the Invention
This invention relates generally to pulmonary performance test equipment, and more particularly to a method and apparatus for calibrating such test equipment when used for measuring single-breath, carbon monoxide lung diffusion, commonly referred to by the symbol DL whereby the accuracy of the test is assured.
II. Discussion of the Prior Art
As is pointed out in an article entitled "Single Breath Carbon Monoxide Diffusing Capacity (Transfer Factor)", published in the 1987 edition of the American Review of Respiratory Diseases, Vol. 136, pp. 1299-1307, considerable difficulty has been experienced in obtaining accurate measurements of the diffusing capacity of a patient's lungs due to variabilities introduced by (1) test technique; (2) errors in gas analysis; and (3) the computation algorithms. A study discussed in the above-referenced article reveals the DL.sub.co measurements can vary between 53% and 125% due to errors in gas analysis. One cannot expect accurate measurements to be made if the equipment used in making those measurements is not properly calibrated.
In the Huhn U.S. Pat. No. 5,119,825, which is assigned applicant's assignee, there is described an apparatus useful in making DL.sub.co measurements. That patent describes a multifunction patient valve and how that valve can be sequenced to allow a preanalysis gas mixture including carbon monoxide, a tracer gas, such as neon or helium and air to be introduced through a mouthpiece into the patient's lungs, held there for a precise time interval, and then exhaled into a patient circuit leading to a sampling chamber. The as in the sampling chamber is then analyzed to determine the amount of the CO that had diffused through the lung tissue into the bloodstream during that precise interval by measuring the concentration of CO and the tracer gas exhaled into the sampling chamber.
The accurate determination of DL.sub.co requires the integration of data from several analyzers and transducers, as well as the coordination of the plumbing and valving which directs the gas flow to and from the patient. For this reason, verification of the performance of the individual components used to determine DL.sub.co does not establish the accuracy of the entire system. In accordance with the present invention, the accuracy of a test system is confirmed by presenting a precise volume of gas concentrations to the entire system in such a way as to simulate the entire DL.sub.co maneuver as it is performed by a patient. In this fashion, the system, as a whole, is tested and calibrated whereas in the past, only the individual components comprising the system had been individually tested in an attempt to provide assurance of system calibration.